Of the available speech digitization techniques, one of the more popular is the waveform follower technique that attempts to emulate the speech waveform. Although the waveform follower technique requires more transmission bandwidth than other techniques, it has been preferred due to its simple implementation architecture and low processing and power requirements. Two types of waveform follower speech signal encoding techniques are pulse code modulation (PCM) and continuously variable slope delta modulation (CVSD). PCM, which is basically a quantized pulse amplitude modulation, obtains an adequate representation of an analog signal by sampling the signal and encoding each sample as an approximation to one of several allowable discrete values. A typical PCM technique samples the analog speech signal 8000 times per second. Each sample is represented by 8 bits for a total bit rate requirement of 64 Kbps.
In contrast, CVSD does not encode each signal sample approximation, but instead encodes discrete increments of the signal, relative to the previous sample approximation. FIG. 1A illustrates a CVSD digital transmission scheme in accordance with the prior art. As shown in FIG. 1A, analog signal 105 is sampled six times, namely t0–t5. The initial value at t0 is the reference value for the next subsequent sample. Subsequent increases in the value of the signal are encoded as a 1, whereas subsequent decreases in the signal are encoded as a 0. At t1 the signal has increases from t0 and therefore a 1 is transmitted. At t2 the signal has decreased from t1 and therefore a 0 is transmitted, and so on. CVSD encoding chart 110 illustrates the encoding of signal 105. The signal can then be reconstructed by increasing the value of the reconstructed signal in response to a 1 being received, and decreasing the reconstructed signal in response to a 0 being received. Because CVSD does not transmit each approximate signal sample, but only a relative change in the signal, CVSD requires a significantly lower bit rate than PCM. A typical CVSD technique requires a bit rate of 32 Kbps. In the radio domain, where bandwidth is a concern, CVSD has been preferred over PCM because CVSD provides equivalent speech quality with approximately half the bit rate requirement. Additionally, for radio, a baseline of about 16 Kbps is typically considered sufficient to provide adequate quality, so CVSD provides more than adequate quality.
The use of CVSD, however, presents a drawback for systems subject to burst errors. From FIG. 1A, it can be appreciated that CVSD depends heavily on previous data to accurately reconstruct a signal. For bust errors (burst interference), an entire packet data, or more, may be corrupted at one time. This means that some previous data, which the CVSD scheme depends so heavily upon, may be lost. CVSD speech encoding is subject to error extension and severe loss of speech quality when subject to losses of packets. FIG. 1B provides an illustration of the effect of a burst error on signal recovery using CVSD. Signal 110 suffers a burst error from time t3–t6. At time t6 the reference to the signal has been established. The reconstruction of the signal using CVSD compares the signal at t3 (0) to the signal t6(−1). Since the value at t6 is lower, the CVSD scheme sends the reconstructed signal lower. The value of the signal at time t7 (0) and the value of the reconstructed signal at time t7, (−2) is now totally distorted. The distortion continues at t8 and t9, and may continue from one packet to another. Due to gaps, typical in speech signals, there is a tendency for the signal to revert to zero periodically which is eventually ends the error propagation.
Systems that employ frequency hopping are prone to burst errors. Frequency hopping may be employed where multiple systems are in use in relatively small area. Each device randomly hops from one frequency to another until a frequency is found that is not in use by some other device at the time. The device may then use the frequency to communicate for a short time before hopping to another available frequency. Thus, the problem of trying to assign a designated frequency for to each device in a dynamic (e.g., mobile) environment is avoided. However, because the hopping is random, there are instances where two or more devices have selected the same frequency causing mutual interference.
The short-range networking protocol Bluetooth is an example of a frequency-hopping system. Bluetooth hops over a frequency band of 2.402 GHz to 2.48 GHz in 1 MHz increments for a total of 79 channels. The Bluetooth protocol provides for frequency hopping at the rate of 1600 hops per second with 64 bits of data in each hop.
Frequency hopping wireless systems operating in a congested RF environment such as Bluetooth may address the problem of interference with a data transmission by requesting a retransmission, however to maintain quality speech transmissions, the delay associated with retransmission must be avoided. Such systems must be able to extrapolate across lost packets.